1. Field of the Invention
The present invention relates to Chemical Vapor Deposition (CVD) for forming a desired film on a substrate using a catalyzer More particularly, the invention relates to a CVD apparatus that uses a catalyzer member for applying a catalysis to a CVD reaction or reactions and that is equipped with a cleaning device for cleaning the inside of the reaction chamber after a CVD process or processes is/are completed, and a film formation method using the CVD apparatus.
2. Description of the Prior Art
In the fabrication process sequence of semiconductor devices, for example, Large-Scale Integrated circuits (LSIs) designed for memories, microprocessors, and so on, various thin films need to be formed on a substrate These thin films include dielectric films, such as a silicon nitride (SiN.sub.x) film which is used for an oxidation-resistant masking film in the isolation-dielectric formation process of Metal-Oxide-Semiconductor (MOS) LSIs, and a silicon dioxide (SiO.sub.2) film which is used for a passivation film. Furthermore, they include conductive films, such as a polysilicon film which is used for forming gate electrodes and gate wiring lines in MOS LSIs, and a tungsten (W) film which is used for forming contact plugs of multilevel wiring structures
To form the above-described thin films, various CVD processes have been developed and extensively used in the semiconductor device fabrication field In these CVD processes, suitable catalyzers may be used to lower the necessary temperature of the substrate and to improve the quality of the films formed on the substrate. Here, these processes are termed "catalytic CVD processes".
In a typical catalytic CVD process, a suitable catalyzer member (which is made of, for example, a refractory metal) is placed in a reaction chamber along with a substrate. The substrate and the catalyzer member are heated to specific temperatures, respectively. Then, suitable gaseous source materials are then supplied to the chamber, thereby forming a desired film on the surface of the substrate through a specific CVD reaction or reactions under the catalysis of the catalyzer member. There is a benefit that the thin film thus formed has a satisfactorily good quality even when the temperature of the substrate is comparatively low.
FIG. 1 schematically shows the configuration of a prior-art catalytic CVD apparatus used for performing a catalytic CVD process.
In FIG. 1, the CVD apparatus is comprised of a reaction chamber 151 made of quartz and a coil-shaped catalyzer member 152 placed in the chamber 151. The catalyzer member 152 is formed by a piece of wire made of a refractory metal such as tungsten (W). The catalyzer member 152 is electrically connected to a power supply 153 placed outside the chamber 151 for heating the member 152 to a specific temperature on operation. A substrate stage 155 on which a single-crystal silicon (Si) substrate 154 is placed is fixed in the chamber 151. The stage 155 is positioned right below the catalyzer member 152.
A shutter 156, which is horizontally movable along the horizontal arrow in FIG. 1, is provided in the chamber 151 between the catalyzer member 152 and the substrate stage 155. The shutter 156 can be positioned at a closing position and an opening position. At the closing position, the shutter 156 is located just over the substrate 154 placed on the stage 155 and entirely covers the surface of the substrate 154. At the opening position, the shutter 156 is located apart from the substrate 154 and entirely exposes the surface of the substrate 154, allowing active species 159 generated in the vicinity of the catalyzer member 152 to reach the substrate 154.
A gas inlet 157 is provided at an upper position of the side wall of the reaction chamber 151. Source or reactant gas or gases SG is/are supplied into the reaction chamber 151 through the gas inlet 157. A gas outlet 158 is provided at the bottom wall of the chamber 151. Gaseous substances existing in the chamber 151 are exhausted to the outside of the chamber 151 through the gas outlet 158.
The above-described prior-art CVD apparatus is used in the following way, in which a thin SiN.sub.x film used as a dielectric in the semiconductor device is formed on the substrate 154.
First, the Si substrate or wafer 154 is sent to the inside of the reaction chamber 151 and is placed on the substrate stage 155. The substrate 154 is then heated up to a specific temperature ranging from 300 to 400.degree. C. and kept at the same temperature by using a heater (not shown) incorporated into the stage 155.
Next, while the shutter 156 is located at the closing position just over the substrate 154, the catalyzer member 152 is heated up to a specific high temperature ranging from 1700 to 1800.degree. C. and kept at the same temperature by using the power supply 153. Thereafter, as the source or reactant gases SG, gaseous monosilane (SiH.sub.4) and ammonia (NH.sub.3) are introduced into the chamber 151 through the gas inlet 157 at their specific flow rates. The introduced SiH.sub.4 and NH.sub.3 are decomposed due to the catalysis of the heated catalyzer member 152, generating the active species 159 in the vicinity of the member 152. Because of the shutter 156 at the closing position, the active species 159 thus generated do not reach the substrate 154 at this stage.
After the flow rates of the gaseous SiH.sub.4 and NH.sub.3 and the temperature of the catalyzer member 152 become steady, the shutter 156 is horizontally moved to the opening position to thereby expose entirely the surface of the substrate 154 to the active species 159, as shown in FIG. 1. Thus, the active species 159 generated from the SiH.sub.4 and NH.sub.3 gases SG begin to be supplied to the surface of the substrate 154, as shown by the vertical arrows in FIG. 1. The active species 159 react with the Si atoms of the substrate 154 and deposit SiN.sub.x on the surface of the substrate 154. After a specific deposition period passes, the shutter 156 is moved to the closing position again, completing the deposition process. Thus, a desired SiN.sub.x film (not shown) with a desired thickness is formed on the surface of the Si substrate 154.
In the prior-art catalytic CVD apparatus shown in FIG. 1, thereafter, the substrate 154 with the deposited SiN.sub.x film is taken out of the reaction chamber 151 and then, a cleaning process is conducted to clean the inside of the chamber 151, i.e., to removed the unwanted SiN.sub.x films deposited on the inner walls of the chamber 151 or the like. This cleaning process is carried out by an unillustrated cleaning device or subsystem. A next CVD process is then conducted in the same reaction chamber 151 in the same way as above.
In popular CVD apparatuses, a cleaning subsystem is equipped for the purpose of cleaning the inside of a reaction chamber. Typically, gaseous carbon tetrafluoride (CF.sub.4) is used as a cleaning gas. After a CVD process is completed, the cleaning gas is introduced into the reaction chamber and then, CF.sub.4 plasma is generated from the gaseous CF.sub.4 using a popular plasma generator. The CF.sub.4 plasma thus generated removes the unwanted SiN.sub.x films existing in the inside of the reaction chamber by etching.
As seen from the above explanation, the prior-art catalytic CVD apparatus shown in FIG. 1 has a problem that the catalyzer member 152 itself is etched by the CF.sub.4 plasma during the cleaning process, resulting in breaking or degradation of the coil-shaped catalyzer member 152. In other words, in the prior-art catalytic CVD apparatus of in FIG. 1, there is a problem that the inside of the reaction chamber 151 is difficult to be cleaned.
Moreover, the prior-art catalytic CVD apparatus of FIG. 1 has another problem that the temperature of the substrate 154 tends to be raised due to the heat radiated from the heated catalyzer member 152 during the deposition process. This is because the catalyzer member 152 is typically placed at a short distance (e.g., 4 cm to 5 cm) from the substrate 154. As known well, the thickness of the deposited SiN.sub.x film is determined mainly by the temperature of the substrate 154 and therefore, the temperature rising of the substrate 154 during the CVD process will cause unwanted thickness fluctuation of the SiN.sub.x film on the same substrate 154.
FIGS. 2 and 3 show another prior-art catalytic CVD apparatus disclosed in the Japanese Patent No. 2,692,326 published in December 1997 (which corresponds to the Japanese Non-Examined Patent Publication No. 3-239320 published in October 1990). This prior-art apparatus is capable of suppressing the effect of radiated heat from a catalyzer member during a deposition or CVD process, solving the latter problem relating the temperature rise of a substrate.
As shown in FIG. 2, a coil-shaped catalyzer member 261 is placed in a reaction chamber 251. The catalyzer member 261 is electrically connected to a power supply (not shown) provided outside the chamber 251 for heating the catalyzer member 261 to a specific temperature on operation. A substrate stage 262 on which substrates 254 are placed is fixed in the chamber 251 The stage 262 is positioned right below the catalyzer member 261. A radiation-shielding member 263 is provided in the chamber 251 between the catalyzer member 261 and the stage 262.
A gas-supplying tube 257 is provided to penetrate the top wall of the reaction chamber 251. A source gas or gases SG is/are supplied through the tube 257 to the inside of the chamber 251. The end part of the tube 257, which is placed in the chamber 251, has small nozzle-shaped holes. The source gas or gases SG is/are vertically emitted through the nozzle-shaped holes into the chamber 151, as shown by the vertical allows in FIG. 2. The catalyzer member 261 is located near and below the holes of the tube 257.
A gas outlet 258 is provided at the side wall of the reaction chamber 251. Gaseous substances existing in the chamber 251 are exhausted to the outside of the chamber 251 through the gas outlet 258.
A heater 271 and a cooling tube 272 are provided in the substrate stage 262. The heater 271 is used to heat the substrates 254 placed on the stage 262 by supplying electric power. The cooling tube 272 is used to cool the substrates 254 placed on the stage 262 by flowing a cooling water through the tube 272. A window 273, through which the inside of the chamber 251 can be seen, is provided at the side wall of the chamber 251.
As shown in FIG. 3, the radiation-shielding member 263 is comprised of a cylindrical member 267, three upper plate members 265a, 265b, and 265c arranged at specific intervals to form slits in a horizontal plane, and lower plate members 266a, 266b, and 266c arranged at specific intervals to form slits in another horizontal plane. These members 265a, 265b, 265c, 266a, 266b, and 266c are formed by elongated stainless-steel plates. The upper plate members 265a, 265b, and 265c are located over the lower plate members 266a, 266b, and 266c at a specific gap. The upper plate members 265a, 265b, and 265c are shifted in a horizontal direction so as to partially overlapped with the lower plate members 266a, 266b, and 266c.
Due to existence of the radiation-shielding member 263, the heat radiated from the catalyzer member 261 is prevented from reaching directly the substrates 254 while allowing the source gas or gases SG or active species to reach the substrates 254 through the slits of the member 263.
With the above-described prior-art CVD apparatus shown in FIGS. 2 and 3, the above-described latter problem about the temperature rise of the substrates 254 can be solved by the radiation-shielding member 263. However, the above-described former problem about the cleaning process is left unsolved.